1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly, to a pneumatic tire in which pattern noise is reduced while other properties (in particular, performance on wet road surfaces) are maintained.
2. Description of the Related Art
Lug grooves, which endow a pneumatic tire with performance on wet road surfaces and resistance to hydroplaning in particular, are indispensable to pneumatic tires.
However, due to the existence of lug grooves, pitch noise (impact noise) is generated at the time the leading (step-in) edge of a block of the pneumatic tire contacts a road surface.
Various studies have been conducted in order to determine methods of reducing the pattern noise generated from the lug grooves (pitch noise being the main type of pattern noise). In particular, pitch variation, transverse direction phase offsetting, and the like have been studied in an attempt to reduce pattern noise.
Generally, there is a correlation between the negative ratio, the sound level, and the performance on wet road surfaces. If the negative ratio is reduced, the sound level improves, but the performance on wet road surfaces deteriorates. If the negative ratio is increased, the performance on wet road surfaces improves, but the sound level deteriorates.
In view of the aforementioned, an object of the present invention is to provide a pneumatic tire in which pattern noise can be reduced without a deterioration in the performance on wet road surfaces.
As illustrated in FIG. 10, impact noise is generated when a tire 100 rotates and a block 102 contacts a road surface 104. (Hereinafter, this impact noise will be called xe2x80x9cpitch noisexe2x80x9d, and the waveform thereof is illustrated in FIG. 11.)
When the pattern of a tread is being designed, the angle of the edge portion of the block is an important factor. Therefore, the present inventors studied the angle of the block edge portion.
Due to the existence of lug grooves, pitch noise is generated when the leading edge of a block contacts the road surface.
It is known that the magnitude of the pitch noise is determined by the angle formed by the tire leading edge side contour line of the ground-contact configuration and the side surface of the block leading edge side.
More specifically, as illustrated in FIG. 12, when an angle xcex8 (hereinafter, xe2x80x9cground-contact angle xcex8xe2x80x9d), which is formed by a tire leading edge side contour line 106 of the ground contact configuration and the tire circumferential direction (the direction of arrow A and the direction of arrow B), is equal to an angle xcfx86 of the leading edge of the block 102 (the angle formed by a side surface 102A of the leading edge side of the block 102 and the tire circumferential direction), i.e., when the tire leading edge side contour line 106 of the ground-contact configuration and the side surface 102A of the leading edge side of the block 102 are parallel (i.e., when xcfx86=xcex8), as illustrated in FIG. 13, the pitch noise is greatest. When the tire leading edge side contour line 106 of the ground-contact configuration and the side surface 102A of the leading edge side of the block 102 are orthogonal (i.e., when the difference between xcex8 and xcfx86 is 90xc2x0), the pitch noise is lowest. (Note that in a case in which the tire leading edge side contour line is curved, as shown in FIG. 12, the ground-contact angle xcex8 is the angle formed by the tire circumferential direction and a tangent line SL which passes through a point tangent to the block 102 leading edge (the end portion which first contacts the ground).)
The angular difference between xcex8 and xcfx86 is important to the reduction of pitch noise.
Here, the relationship between the tire leading edge side contour line 106 of the ground-contact configuration and the side surface 102A of the leading edge side of the block is considered.
First, in a case in which blocks are provided at the left and right of the tire equatorial plane, the angles at the respective portions are set as illustrated in FIGS. 14A and 14B. Namely, with respect to a block 102R at the right side of a tire equatorial plane CL, the angles are defined in the clockwise direction. The angle at the block leading edge is xcfx861, and the ground-contact angle formed by the tire circumferential direction and the tire leading edge side contour line 106 of the ground-contact configuration is xcex81.
On the other hand, with respect to a block 102L at the left side of the tire equatorial plane CL, the angles are defined in the counterclockwise direction. The angle at the block leading edge is xcfx862, and the ground-contact angle formed by the tire circumferential direction and the tire leading edge side contour line 106 of the ground-contact configuration is xcex82.
The positional relationships among the tire leading edge side contour line 106, the side surface 102A of the leading edge side of the block 102R at the right side of the tire equatorial plane CL, and the side surface 102A of the leading edge side of the block 102L at the left side of the tire equatorial plane CL, are as follows.
Assuming that xcex2 greater than 0xc2x0, xcex11 greater than 0xc2x0, and xcex12 greater than 0xc2x0, then xcex81=90xc2x0+xcex2, xcex82=90xc2x0+xcex2, xcfx861=90xc2x0xe2x88x92xcex11, and xcfx862=90xc2x0+xcex12.
As described above, the angular difference between the ground-contact angle xcex8 and the angle xcfx86 of the block leading edge is important to pitch noise. The angular difference "THgr"1 of the block 102 at the right side of the tire equatorial plane CL is "THgr"1=xcex81xe2x88x92xcfx861=xcex2+xcex11, and the angular difference "THgr"2 of the block 102 at the left side of the tire equatorial plane CL is "THgr"2=xcfx862xe2x88x92xcex82=xcex12xe2x88x92xcex2. The relationship between the angles and the magnitude of the pitch noise is as shown in FIG. 15.
FIG. 15 illustrates that pitch noise of a magnitude P1 is generated from the block 102 at the right side of the tire equatorial plane CL, and pitch noise of a magnitude P2 is generated from the block 102 at the left side of the tire equatorial plane CL. ("THgr"2 less than "THgr"1, and therefore, the magnitudes of the pitch noise are P2 greater than P1.)
One conventional method of reducing pitch noise centers around the tire transverse direction phase offsetting of blocks. In the present invention as well, the phases of the left and right blocks are offset by a dimension D in the tire circumferential direction.
By providing a phase difference for respective pitch noises generated from block rows of blocks (generally, pairs of left and right blocks with respect to an axis extending along the tire circumferential direction (e.g., the tire equatorial plane CL)), the sounds can cancel each other out. The necessary extent of the phase difference differs in accordance with the configurations or the like of respective tires, and is determined for each tire.
As illustrated in FIGS. 16A and 16B, two sounds (Â and {circle around (B)}) are completely reverse phases. When the magnitude of the amplitude Pa and the magnitude of the amplitude Pb are equal, the magnitude of the combined sound is a minimum (FIG. 16B). However, when there is a difference between the amplitudes, the magnitude of the combined sound is not zero, and a sound having a magnitude of an amplitude |Paxe2x88x92Pb| remains (see FIG. 16A).
It can thus be understood that, in order to make the phase offsetting effect a maximum, the magnitudes of the amplitudes of the sounds generated by the respective subject blocks must be equal.
Here, in FIGS. 14A and 14B, the angles of inclination of the lug grooves are equal, i.e., the side surface 102A of the leading edge side of the block 102L at the left side of the tire equatorial plane CL and the side surface 102A of the leading edge side of the block 102R at the right side of the tire equatorial plane CL are substantially parallel (i.e., xcex11≈xcex12). The angular difference "THgr"1 of the block 102R at the right side of the tire equatorial plane CL and the angular difference "THgr"2 of the block 102L at the left side of the tire equatorial plane CL are not the same. Sounds of different magnitudes are generated at the respective sides of the tire equatorial plane CL. Even if a block positional relationship is set in which the left and right blocks are offset in the tire circumferential direction so that the sounds at the respective sides become have reverse phases, a sound having the amplitude (P2xe2x88x92P1) remains.
Accordingly, in order to make the sounds from the left and right blocks to be the same magnitude and to exhibit the maximum phase offset effect, it is necessary for the angular difference "THgr"1=the angular difference "THgr"2.
Here, there are several conditions which satisfy the angular difference "THgr"1=the angular difference "THgr"2.
To briefly explain by using the example of FIGS. 14A and 14B, it suffices for xcex2+xcex11=xcex12xe2x88x92xcex2. Here, xcex2 is an angle determined unambiguously from the ground-contact configuration, and a is an angle selected arbitrarily (an angle which can be changed by changing the configuration of the block). For example, if xcex12 is fixed and xcex11 is made small, or if xcex11 is fixed and xcex12 is made large, the equation xcex2+xcex11=xcex12xe2x88x92xcex2 is established. The magnitude of the pitch noise generated if xcex12 is fixed and xcex11 is made small is P2, and the magnitude of the pitch noise generated if xcex11 is fixed and xcex12 is made large is P1.
From the standpoint of phase offsetting, both the method of fixing xcex12 and making xcex11 small and the method of fixing xcex11 and making xcex12 large are the same. However, it can be understood that it is preferable to select the method of fixing xcex11, whose pitch noise is small, and making xcex12 large in both cases.
One aspect of the present invention is a pneumatic tire comprising: a first block row in which a plurality of blocks projecting from an outer circumference of the pneumatic tire are disposed along a tire circumferential direction; and a second block row in which a plurality of blocks projecting from the outer circumference of the pneumatic tire are disposed along the tire circumferential direction, the second block row being parallel to the first block row, wherein a side surface of a leading edge side end portion of each block of the first block row and the second block row is inclined with respect to a tire transverse direction such that an angle, which is formed by the side surface of the leading edge side end portion of each block of the first block row and a tire leading edge side contour line of a ground-contact configuration, and an angle, which is formed by the side surface of the leading edge side end portion of each block of the second block row and the tire leading edge side contour line of the ground-contact configuration, are substantially equal. Therefore, the pitch noise generated at the time of step-in at the blocks of the first block row is substantially the same level as the pitch noise generated at the time of step-in at the blocks of the second block row.
Therefore, by adjusting the tire circumferential direction phases of the blocks of the first block row and the blocks of the second block row, the pitch noises of substantially the same level will interfere with one another and cancel out one another, and the pattern noise of the tire can be reduced.
In the present invention, because there is no need to change the negative ratio, the performance on wet road surfaces and operational stability do not deteriorate.
In another aspect of the present invention, in the pneumatic tire of the previously-described aspect, given that the angle formed by the side surface of the leading edge side of each block of one block row and the tire leading edge side contour line is "THgr"1 and the angle formed by the side surface of the leading edge side of each block of the other block row and the tire leading edge side contour line is "THgr"2, the relation |"THgr"2xe2x88x92"THgr"1|xe2x89xa65xc2x0 is satisfied.
As a result of investigating the relationship between the value of "THgr"2xe2x88x92"THgr"1 and the pitch size by changing the angle xcfx86 of the block leading edge to various values, the results illustrated in FIG. 17 were obtained. From FIG. 17, it is clear that by making |"THgr"2xe2x88x92"THgr"1|xe2x89xa65xc2x0, the pattern noise of the pneumatic tire could be reduced sufficiently.
In the pneumatic tire of this aspect, |"THgr"2xe2x88x92"THgr"1|xe2x89xa65xc2x0, wherein "THgr"1 is the angle formed by the tire leading edge side contour line and the side surface of the leading edge side of a block of one block row, and "THgr"2 is the angle formed by the tire leading edge side contour line and the side surface of the leading edge side of a block of another block row. Therefore, the pattern noise of the tire can be reliably decreased.
It is even more preferable that |"THgr"2xe2x88x92"THgr"1|xe2x89xa62xc2x0.